The invention relates to an injection molding device comprising a mold body having a cavity, an elongated nozzle seated in the cavity, a valve pin coaxially in the nozzle and actuating means connected to the valve pin for axially displacing the valve pin in the nozzle, the actuating means comprising a cylinder housing having a first and a second pressure medium inlet, and a piston reciprocatable in the cylinder housing, the cylinder piston being coupled to the valve pin.
A valve gated injection molding device of the above-mentioned type is known from EP-A-0 836 925 in the name of the applicant. By axial displacement of the valve pin, the gate of the nozzle is opened and closed to control the flow of molten thermoplastic material into a mold cavity. The valve pin head projects from the upper part of the nozzle and is seated in a sliding bush which is axially displaced by means of a lever arm. The lever arm is connected to a hydraulic cylinder that is placed at a radially offset location from the valve pin. The known valve pin actuator has as a disadvantage that it requires a relatively large space outside the injection molding body, and that a relatively complex lever system is used for coupling the piston to the valve pin head. Especially in injection molding devices wherein the nozzles are to be located in close proximity to a central feed of thermoplastic material in the manifold, it is required for the actuating cylinders to be relatively compact, while at the same time providing a sufficient stroke for operation of the valve pin. Furthermore, access to the valve pin head is relatively cumbersome and may require taking a part of the mould body apart.
It is an object of the present invention to provide a valve gated injection molding device, having a valve pin actuator of relatively compact dimensions which is of a simple construction. It is a further object to provide a valve pin actuator, which can be easily detached from the injection molding device for maintenance, or replacement,
Hereto the injection molding device according to the invention is characterised in that the cylinder is placed above the valve pin, coaxial therewith, the cylinder housing being detachably coupled to a base member on the mold body, and defining a chamber with an upper part having a first diameter, the piston having a piston head which is in contacting relationship with the walls of the upper chamber part, the first and second pressure medium inlets connecting to the upper chamber part above and below the piston head, respectively, and a lower chamber part comprising height adjustment means that are axially displaceable in the cylinder housing, the height adjustment means comprising a bore having a second diameter, smaller than the diameter of the upper chamber part, the piston comprising a stem being slidably seated in the bore of the height adjustment means, the valve pin being guided through a bore in the base member and through the bore of the height adjustment means and being with a valve pin head releasably attached to the piston.
By placing the actuating cylinder directly above the valve pin head, no complex transmission from the piston to the valve pin is required. The linear motion of the piston is transmitted directly to the valve pin. The cylinder according to the invention can easily be installed, and a fine-tuning of the stroke of the piston is effected via the height adjustment means. Removal of the cylinder can be effected by decoupling the cylinder housing followed by detaching the valve pin head from the piston, and removal of the piston and the height adjustment means while the valve pin remains sealed in the flow channel of the nozzle. Not removing the valve pin from the nozzle has as an advantage that only a low headspace is required to remove the actuating cylinder and that the risk of damage to the valve pin is minimised. Also, the rotational position of the valve pin is maintained upon removal of the cylinder. Furthermore, removal of the cylinder according to the invention is facilitated by the fact that no adhesive forces of the solidified thermoplastic material sticking to the needle need to be overcome, which normally occur when removing the valve pin from the nozzle.
An embodiment of the injection molding device according to the invention is characterised in that the piston stem comprises an axial slot, extending to the perimeter of the stem, the slit having a supporting shoulder engaging with a complementary shoulder on the valve pin head, the valve pin head being radially removable from the stem.
The valve pin head is easily removed from the piston by radial displacement of the piston, after removal of the housing, such that the valve pin head is released and the piston and the height adjustment means can be removed. The slot may have a generally T-shaped cross-section. In an embodiment, the valve pin head comprises a flat section which is received in the narrow part of the T-shaped slot in a fixed angular position. In this way the valve pin head is fixed in the proper angular orientation by the piston, which is required when the nozzle tip has a non-rotationally symmetric configuration and a contoured valve pin tip is used.
In a preferred embodiment, the cylinder housing is placed on a cooled base plate. In this way, the interior of the cylinder is thermally insulated from the heated mold body, the temperature of which may for instance be about 200xc2x0 C.-350xc2x0 C. By the cooled base plate, the internal seals of the cylinder are not exposed to high temperatures, such that the service life is increased.